Polyfidelity

Polyfidelity (also sometimes called Polyexclusivity) is a form of polyamory, where all members are considered equal partners and agree to have an active sexual life only with other members of the group. The term originated in the Kerista commune in the Haight-Ashbury neighborhood of San Francisco, California, where polyfidelity was practiced from 1971 to 1991.

These types of relationships are, like monogamous relationships, “closed,” in the sense that partners agree not to have sexual relations outside of current group members. The difference is that more than two people are included in the closed group. New people can be added to the group by unanimous consensus of existing members, or the group may not accept new members.

Prior to the Kerista experience, people would probably have called this arrangement a "complex marriage." In the middle of the 19th century, group marriage in the Oneida Community implied having several partners, but not necessarily the duty to be with all the members of the commune. It was also known as a "group marriage." The modern and broader term polyamory was coined later, in the early 1990s.